In a large pot, combine the potatoes, leeks, garlic, broth, cilantro, curry powder, cumin, salt and pepper. Cook, tightly covered, over medium heat for about 30 minutes or until potatoes are soft. Puree potato mixture either in a blender, small batches at a time, or right in pot, using a hand-held immersion blender. Add the butter and 2 cups of the milk to pot, along with potato puree. If mixture seems too thick, add additional milk. Adjust seasonings, reheat and serve.